1. Field of the Invention
The present invention relates to a digital servo apparatus. More specifically, it relates to a digital servo apparatus which controls the phases and speeds of motors for, e.g., video tape recorders.
2. Description of the Prior Art
Digital servo apparatuses have been utilized for controlling rotation of motors provided in, e.g., video tape recorders. As shown in Matsushita Technical Report, Vol. 28, No. 3, June 1982, FIGS. 25 and 26 (pages 191 and 192), a conventional digital servo apparatus comprises three counters respectively serving as a speed controller, a phase controller and a frequency divider for generating phase reference signals, with respect to one motor.
FIG. 1 is a schematic block diagram showing a conventional digital servo apparatus. Referring to FIG. 1, description is now made on the conventional digital servo apparatus. With respect to a motor 1, provided are a speed detecting pulse generator 1a and a phase detecting pulse generator 1b. First and second counters 2a and 2b respectively count clock pulses generated from a clock pulse generator 3, the first counter 2a being reset by an FG pulse generated by the speed detecting pulse generator 1a and second counter 2b being reset by a PG pulse generated by the phase detecting pulse generator 1b. Count outputs from the first counter 2a are converted into analog signals by a first D-A converter 3a, to be sampled and held by a first sample-and-hold circuit 4a and supplied to an adder 5.
On the other hand, count outputs from the second counter 2b are converted into analog signals by a second D-A converter 3b, to be sampled and held by a second sample-and-hold circuit 4b and supplied to the adder 5. The adder 5 adds the signals respectively sampled and held by the first and second sample-and-hold circuits 4a and 4b and applies the added signal to a driving circuit as error signals. The driving circuit 7 controls the speed and phase of the motor 1 on the basis of the supplied error signals.
FIGS. 2 and 3 are timing charts for illustrating the operation of the digital servo apparatus as shown in FIG. 1 for controlling the speed and phase of the motor.
The operation of the conventional digital servo apparatus is now described with reference to FIGS. 1 to 3. The first counter 2a counts the reference clock signals generated by the reference clock signal generator 3, the count output of which is gradually increased as shown in FIG. 2(A). The count output from the first counter 2a is supplied to the first D-A converter 3a in synchronization with an FG pulse generated from the speed detecting pulse generator 1a. The first counter 2a is reset by the FG pulse. The analog signal converted by the first D-A converter 3a is sampled and held by the first sample-and-hold circuit 4a to the rise of a subsequent FG pulse, and supplied to the adder 5 as a speed error signal.
As shown in FIG. 3(B), the second counter 2b starts counting the reference clock signals from the reference clock signal generator 3 in synchronization with a V pulse generated from the phase reference signal generator 6. The count output from the second counter 2b is gradually increased as shown in FIG. 3(A). The said count output from the second counter 2b is supplied to the second D-A converter 3b in synchronization with a PG pulse generated from the phase detecting pulse generator lb , as shown in FIG. 3(C). Thereafter the second counter 2b is reset. The analog signal outputted from the second D-A converter 3b is sampled and held to the rise of a subsequent PG pulse, and supplied to the adder 5 as a phase error signal. The outputs from the adder 5 are supplied to the driving circuit 7, to control the speed and phase of the motor 1.
In case of forming the digital servo apparatus in the aforementioned structure by a one-chip microcomputer, however, such a microcomputer generally contains only one or two counters in one chip, and hence the apparatus formed by the same can merely control the speed and phase of only one motor by one chip at the most.
On the contrary, a video tape recorder generally contains a plurality of motors such as a capstan motor in addition to a cylinder motor, and the number of microcomputers must be increased in order to control the motors.